Certain designs of power amplifiers (PAs) of a transmitter chain in a communication system employ non-constant envelope modulation techniques to improve the bandwidth efficiency. In order to preserve the signal shape, the non-constant envelope modulation techniques require linear amplification. However, linear amplification is possible only when the power amplifier is operated with a small input signal, which in turn makes the amplifier least efficient. The nonlinear characteristic of the power amplifier makes it difficult to achieve power efficiency and bandwidth efficiency simultaneously.
Many techniques have been developed to improve the linearity of the PA in the transmitter chain. Three main categories of these techniques are feedforward, feedback, and predistortion.
Feedforward techniques are more expensive and less efficient than feedback techniques. The feedback linearization technique can be applied directly around the RF amplifier or indirectly upon the modulation.
Two well-known feedback techniques are Cartesian feedback and polar-loop feedback. The Cartesian feedback technique involves the suppression of nonlinearity in a complex-baseband, expressed by using rectangular Cartesian coordinates, so that it can reduce the shortcomings of quadrature modulators. However, it requires a large amount of memory to store the table of 2-dimensional coefficients for the I-Q data. Another disadvantage of the Cartesian feedback technique is the difficulty in implementing local phase control for the quadrature modulator and demodulator.
In the polar-loop feedback technique, the RF signal is directly generated by a voltage-controlled oscillator whose phase is controlled by feedback signals and whose amplitude is modulated by the difference between a reference signal and a feedback signal. The issues that complicate the design of polar-loop feedback are as follows: First, it requires a precision receiver within the transmitter; second, the control loop bandwidth greatly exceeds the signal bandwidth; third, it restricts the dynamic range of output power and lastly, the maintainability of the stability of the feedback loop is complex tricky.
As such, what is desired is a system and method for improving the transmission linearization that costs less and is easy to implement.